The present invention relates to digital memory systems and more particularly to cross-tie memory systems which store data in the form of stable magnetic states.
The dipole magnetic moments of neighboring atoms within a small region, or domain, of a thin film of magnetic material align themselves when placed in an external magnetic field. This alignment of magnetic dipole moments is unique to magnetic materials (Fe, Co, Ni, Gd and Dy), and takes place despite the random motion generally undergone by atoms within any material. The magnetic orientation of the magnetic material remains after the external magnetic field is removed.
Transition regions exist between any two domains which do not have the same alignment of magnetic dipoles. The transition regions between such domains are called domain walls. Different types of domain walls exist, each unique as to the orientation of the magnetization existent within or comprising the domain wall. Within a domain wall referred to as a Neel wall, the magnetization rotates within the plane of the magnetic film. The magnetization in a domain wall referred to as a Bloch-wall rotates out of the plane of the film. Reversing the magnetization direction in a small portion of a Neel wall results in the creation of a third type of domain wall, the cross-tie. The cross-tie magnetization opposes the magnetization in the domains separated by the Neel wall.
Associated with each cross-tie is, as mentioned, a section of reversed or inverted Neel wall; the inverted Neel wall section is bordered by the cross-tie on one end and a Bloch-wall, or a Bloch-line in a very thin film, on the other end.
The characteristic magnetization of the domain wall types remain unchanged in the absence of an external magnetic field of a predetermined strength. In the presence of an external field of such predetermined strength, however, the magnetic state of a domain wall, i.e., the domain state, of the magnetic film at any given location may be changed.
The stable magnetic domain states of the magnetic film represented by the domain wall magnetization directions may be utilized within a random access memory system for the storage of data. Such a memory system is referred to as a "cross-tie" memory system. In a binary cross-tie memory system the stable Neel wall state and the alternative reversed Neel wall with a cross-tie and Bloch-line pair state may be used to represent a bit of data. The data can be written to memory by the application of an appropriate magnetic field. Where a Neel wall exists, a cross-tie Bloch-line pair may be introduced by the application of the appropriate magnetic field. An opposite field may be used to restore the original Neel wall domain state.
The data can also be read from the magnetic memory. The read-out may be accomplished by the use of magneto-resistive effects. While introducing small magnetic fields into the domain walls of the magnetic film, the resistance within the film can be measured. The resistance within the film changes with the introduction of magnetic fields. The precise change in resistance which takes place upon the introduction of magnetic fields varies according to the domain state of the magnetic film. The resistance change is small if the domain state is a Neel wall and, the resistance change is larger if the cross-tie state exits. A measurement of the resistance reveals the state of the domain wall and thus a digit of stored data. The precise amount of the resistance measured in each state and the measured resistance change differs according to which of the known cross-tie memory systems is being practiced.
Reading and writing of individual domains within the magnetic film of such a system is accomplished by means of conductors aligned over positioned domain walls. The currents through the conductors create the small and larger magnetic fields which respectively allow magneto-resistive reading and perform the writing of data.
Although the change in resistance measured when reading is larger for one of the above-mentioned states, the overall resistance change for either state is extremely minute and very difficult to detect, e.g., on the order of 10 to 300 milliohms for the cross-tie state. The minuteness of the resistance change results from the minuteness of the regions containing the domain walls. The thin film material is on the order of 500 Angstroms or less in thickness and the field regions or memory locations are only a few microns wide.
For memory system applications, there is, in addition to the problem of small data signal detection, the problem caused by offset signals which result in a decrease of the signal to noise ratio of the system and the problem caused by non-uniform noise.
Heretofore, various schemes have been proposed for alleviating the read-out problems associated with the minuteness of data signals. For example, alternative domain configurations for increasing the strength of a single field magnetization within each domain location have been proposed to enhance the detection of the change in resistance. Also, methods for improving discrimination between small data signals and potentially larger noise and offset signals have been proposed.
As taught in U.S. Pat. No. 3,868,660, the thin magnetic film may be formed with spaced serrated edges. The serrated-strip configuration provides the means whereby single Neel wall sections occur at predetermined sections along the strip, the occurrence of the domain walls being dependent in part on the width of the strip. U.S. Pat. No. 4,075,612 teaches rounded serrations which put the physical geometry of the strip in substantial alignment with the natural contour of the expected magnetization. The coincidence of the magnetization lines and the geometry of the film permits some enhancement of the magnetization.
Additionally, noise and other system signal irregularities may be partially minimized as against true data signals by the use of various sampling techniques. In U.S. Pat. No. 4,035,629 for example, correlated double sampling may be used. With correlated double sampling, the read signals are differenced from reference signals in two stages of the read sequence thereby correcting incorrect input which results when the value of the data signals is smaller than the value of the non-uniform noise and offsets.
The heretofore proposed structures for dealing with the problem of detecting small signals when reading the stored magnetic domain configurations, while adequate for the purposes intended, have not specifically brought about a multiple enhancement of the magnetic domain configuration within memory locations and hence of the data signals. That is, the heretofore proposed structures either seek to maximize the signal created by a single domain configuration or to discriminate between a small signal and noise or system-matching offset signals. These structures do not attempt to multiply the magnetic domain configurations present at an individual memory location.